Along with performance improvement of the computer systems, a technique of curtailing a cost by consolidating processing that is distributed to plural servers into a single server has been put in practical use. As a technique of consolidating the servers, a server partitioning is known. This is to make plural operating systems work on a single server by assigning one operating system to each of divided partitions.
The server partitioning is divided into two: a physical partitioning of assigning an operating system to a physical computer resource of a node unit that includes a processor and a memory; and a logical partitioning of virtualizing a physical processor and a memory to generate an arbitrary number of logical partitions (logic processors) in a computer by firmware, called a hypervisor.
Since with the physical partitioning, the operating system can exclusively use physical computer resources, it can attain high performance. However, in order to increase the physical partitions, it is necessary to also multiply physical computer resources, and therefore there is a physical limit to the number of the physical partitions.
In the logical partitioning system, each operating system (guest OS) is made to execute on the logic processor that the hypervisor provides. Moreover, by the hypervisor mapping plural logical partitions on the physical processor, a partition can be divided into units finer than the node. This enables plural logical partitions to be executed by a single physical processor while the processor is changed over in timesharing, and it is possible to generate more of logical partitions than the number of the physical processors and to make them execute simultaneously.
As a method of further making effective use of the physical computer resource in the computer system that combine these physical partitioning and logical partitioning, there are known a method whereby logical partitions share an IO slot by switching over the IO slot that is used among the logical partitions in time sharing without causing degradation of performance (JP-A No. 122640/2005), a method whereby a logical partition that is located on a certain physical partition is migrated to a different physical partition according to a user's policy (JP-A No. 244481/2006), and a method whereby plural servers share plural computer resources through a switch (Multi-Root I/O Virtualization and Sharing Specification Revision 1.0, 1.2.3. MR-IOV Topology pp. 21-23 (May 12, 2008)).
Moreover, as the logical partitioning using the hypervisor, there are a method using only virtualization software and a method using hardware for some processing. The method using the virtualization software has high flexibility over IO virtualization because the virtualization software performs conversion from logical IO resource IDs (indicating a PCI bus number, an MMIO address, an IO address, and a configuration address that are necessary to access the IO device) to physical IO resource IDs. But its performance decreases because of overhead by software processing.
On the other hand, since the method using the hardware can transparently accesses the IO device by converting the logical IO resource ID to the physical IO resource ID required at the time of an IO access, the method has higher performance than the method using only the virtualization software, but its flexibility over the IO virtualization lowers.
As described above, since, in the virtualization method using the hardware, the IO device is provided transparently to the guest OS, its performance is high compared with the method using the virtualization software, but its flexibility over the IO virtualization lowers. This is because, when migration of the logical partition described in JP-A No. 244481/2006, etc. is performed, disagreement is caused between the logical IO resource ID of the IO device that the guest OS uses and the physical IO resource ID.
For example, in the computer system using the virtualization method by the hardware, when Hot Plugging is done, the reallocation of the IO resource ID of the each IO device is performed by hot-plugging a new IO device, and the IO resource ID of the IO device already in use may change. In this case, the guest OS may use the IO resource ID before alteration with an intention of accessing the IO device that was used originally, and may access the different IO device. In the worst case, the computer system may become unable to be activated at the time of reboot because the IO resource ID of a boot device has changed.
Moreover, when live migration of the logical partitions is performed in a similar computer system, an IO configuration of the computer system when seen from the guest OS may be different before and after the migration. In this case, contention arises between the IO resource ID that the guest OS used before the migration and the IO resource ID of another device currently used at the migration destination.